Multi-axially forged piston

ABSTRACT

A multi-axially forged mono-bloc piston includes a lower crown part forged of steel and including a pair of pin bosses and an integral skirt formed as one piece with the pin bosses  20.  The lower crown part is friction welded to an upper crown part to form at least one closed oil gallery within the piston head. The lower crown part is forged in the longitudinal direction of the piston as well as laterally to impart longitudinal and laterally forged features of the piston. The laterally forged features may comprise undercut recesses formed in the piston skirt to reduce material in the overall piston structure. The lateral forging may also result in the piston skirt portion being discoupled from the piston head by laterally forged slots formed in the lower crown part  14  between the skirt and head.

BACKGROUND OF THE INVENTION

[0001] 1. Technical Field

[0002] This invention relates generally to pistons for heavy duty dieselengines, and to the method of making such pistons. The disclosureincorporates the multi-axially forged piston disclosed in provisionalpatent application 60/241,759, filed Oct. 18, 2000, whose priority isclaimed for this application.

[0003] 2. Related Art

[0004] Pistons for heavy duty diesel engine applications are typicallyeither of a monobloc construction in which the piston head, pin bossesand skirt are cast of a single piece, or of an articulated constructionin which the piston skirt is formed separately from the rest of thepiston and joined thereto an articulated fashion through the wrist pin.

[0005] U.S. Pat. No. 4,581,983 discloses a piston of monoblocconstruction wherein the upper part is forged and the lower part is castand joined at an inner face by welding with charged carrier rays with aninter layer of nickel.

[0006] U.S. Pat. No. 3,654,840 discloses a one-piece forged pistonformed in a uni-axial forging process.

[0007] U.S. Pat. No. 4,910,093 discloses a method of forging a one-piecepiston blank of an articulated piston. This reference has no teachingswith respect to forging monobloc pistons wherein the piston skirt is anintegral part of the piston structure, rather than being a separatelyformed, articulated component.

SUMMARY OF THE INVENTION

[0008] According to the invention, a method is provided for forging atleast a portion of the piston head, a pair of pin bosses extendingdownwardly from the piston head, and a piston skirt formed as one piecewith the pin bosses including a pair of opposed skirt portions spacedfrom the pin bosses and intervening strut portions extending between andjoining the skirt portions to the pin bosses as a one piece structure.

[0009] According to the method of the invention, the above features areformed by die-forging a blank of steel in a first axial direction. Thepiston article is further forged in a second axial direction which istransverse to the first axial direction to produce a forged recess ineach of the strut portions of the piston skirt.

[0010] The invention has the advantage of providing a method of forgingat least a lower crown part of a monobloc piston using a multi-axialforging process which minimizes the bulk of material, and thus weight ofthe piston as compared to uni-axially forged pistons. The materialdisplaced in the transverse forging step forms other parts of the pistonstructure in the die, thus decreasing the amount of material needed tomanufacture forged monobloc pistons.

[0011] The invention further is directed to forged monobloc pistons madeby such a multi-axial forging process, wherein at least the lower partof the piston has the skirt and pin bosses formed as one piece andforged in the longitudinal direction as well as in a lateral directionto provide recesses in the strut portions that connect the skirtportions to the pin bosses. A piston constructed in such manner sharesthe same advantages as described above with respect to the multi-axialforging method.

THE DRAWINGS

[0012] Presently preferred embodiments of the invention are disclosed inthe following description and in the accompanying drawings, wherein:

[0013]FIG. 1 is a perspective elevation view of a piston constructedaccording to a first embodiment of the present invention;

[0014]FIG. 2 is a cross-sectional view taken along lines 2-2 of FIG. 1;

[0015]FIG. 3 is a cross-sectional view taken along lines 3-3 of FIG. 1;

[0016]FIG. 4 is a perspective view of a lower forged portion of thepiston;

[0017]FIG. 5 is another perspective view of the lower forged portion;

[0018]FIG. 6 is a front elevation view of the lower forged portion;

[0019]FIG. 7 is a side elevation view of the lower piston portion ofFIG. 6;

[0020]FIG. 8 is a cross-sectional view of an alternative embodiment ofthe invention;

[0021]FIG. 9 is a cross-sectional view taken along lines 9-9 of FIG. 8;and

[0022]FIG. 10 is a perspective view of a piston constructed according toa third embodiment of the invention.

DETAILED DESCRIPTION

[0023] A monosteel piston constructed according to a first embodiment ofthe invention is indicated generally at 10 in FIGS. 1-7 and comprises anupper crown part 12 fabricated of steel and a lower crown part 14. Thelower crown part 14 is forged of one piece of steel and includes atleast a portion 16 of a head 18 of the piston 10, a pair of pin bosses20 extending downwardly from the head portion 16 to lower free ends 22thereof, and a piston skirt 24 formed as one piece with the pin bossesof the same forged material. The skirt includes a pair of skirt portions26 which are arranged on laterally opposite sides of the piston inspaced relation to the pin bosses 20, and intervening strut portions 20extending between and interconnecting the skirt portions 26 and pinbosses 20.

[0024] The piston 10 includes a longitudinal axis A and a pin bore axisB transverse and preferably perpendicular to the longitudinal axis A.The pin bosses 20 are formed with pin bores 30 aligned along the pinbore axis B.

[0025] The lower crown part 14 is forged separately from the upper crowpart 12. While the upper crown part 12 is preferably forged, but couldbe formed by other techniques such as casting. The upper crown part 12has a circumferentially continuous upper wall 32 surrounding adome-shaped bowl wall 34 defining a recessed combustion bowl 36 at thetop of the piston 10. An outer wall or ring belt 38 extends downwardlyfrom the upper wall 32 in radially outwardly spaced relation to thecombustion bowl wall 34. The ring belt 38 is formed with at least oneand preferably a pair of combustion ring grooves 40. The ring belt 38extends to a lower circumferentially continuous joining surface 42.

[0026] A circumferentially continuous inner wall 44 of the upper crownpart 12 extends downwardly from the combustion bowl wall 34 in radiallyinnerwardly spaced relation to the outer wail 38. The inner wall 44presents a circumferentially continuous joining surface 46 at its lowerend. The joining surfaces 42,46 are preferably non-coplanar such thatthe inner wall joining surface 46 resides in a plane lower than thejoining surface 42 of the outer wall 38 as illustrated in FIGS. 2 and 3.The inner wall 44 is preferably parallel to the outer wall 38 in thelongitudinal direction and concentric with respect to the longitudinalaxis A of the piston 10. The combustion bowl wall 34 extends upwardlyfrom the inner wall 44 to a peak at the longitudinal axis A to providean arched combustion bowl structure with peaked combustion bowl walls 34supported by the longitudinally extending circumferentially continuousinner wall 44 extending parallel to the longitudinal axis A.

[0027] The lower crown part 14, as mentioned, is formed as a separateforged component from the upper crown part 12. The lower crown part 14includes at least an inner circumferentially continuous joining surface48 provided at the upper end of a circumferentially continuous innerwall 50 extending upwardly from and joined to the pin bosses 20 incoaxial alignment with the joining surface 46 of the inner wall 44 ofthe upper crown part 12.

[0028] The piston skirt 24 of the lower crown part 14 in the firstembodiment presents a circumferentially continuous upper joining surface52 that is spaced radially outwardly of the joining surface 48 of theinner wall 50 and coaxially in line with the joining surface 42 of thering belt 38. The joining surfaces 48,52 are preferably longitudinallyoffset in different planes to compliment the offset upper crown partjoining surfaces 46,42, respectively. As such, the joining surface 52 ofthe skirt 24 is disposed in a plane axially above the plane of the innerwall joining surface 48, and the corresponding inner joining surfaces46,48 and outer joining surfaces 42,52 of the upper and lower crownparts 12,14, respectively, come together in mutually engagingrelationship.

[0029] The piston skirt 24 of the lower crown part 14 of the firstembodiment is preferably formed with at least one ring groove 54 forreceiving an oil ring of the piston 10. A circumferentially continuouschannel 56 is forged in the lower crown part 14 and extends into theskirt 24 in the space between the inner wall 50 and an outer skirt wall53 of the skirt 24. A floor 58 of the channel 56 is spaced below thejoining surfaces 48,52 and preferably below the ring groove 54 formed inthe skirt wall 53.

[0030] According to the invention, the upper and lower crown parts 12,14are friction welded together, such that the parts 12,14 are joined bypermanent friction weld joints 60,62 across their mating inner 46,48 andouter 42,52 joining surfaces, respectively. When joined, the upper andlower crown parts 12,14 define at least one circumferentially continuousclosed oil gallery 64 radially between the adjoined inner walls 44,50and outer walls 38,53 of the upper and lower crown parts 12,14,respectively.

[0031] According to the first embodiment, the lower crown part 14 isfurther formed with an inner gallery floor 66 spanning the space betweenthe inner wall surfaces 50 and enclosing, when the upper and lower crownparts 12,14 are joined together, an inner oil gallery 68. The innergallery floor 66 is likewise forged in the lower crown part 14 duringformation of the one piece lower crown part. The outer oil gallery 64communicates with the inner oil gallery 68 through one or more ports 70,and the inner oil gallery 68 communicates with the space between the pinbosses 20 through an opening 72 formed in the gallery floors 66. One ormore drain holes 76 are forged in the lower part 14. The drain holes 76are recessed in the outer surface of the skirt wall 53 and openlongitudinally into the ring groove 54 from below to provide oildrainage to the groove 54 during operation of the piston 10.

[0032] The lower crown part 14 is preferably fabricated by a multi-axialforging operation, wherein a lower crown part blank is forged both inthe longitudinal axis A of the lower crown part 12 and also in adirection transverse to the axis A to impart transversely forgedfeatures in the lower crown part 14 that cannot be attained by forgingin the longitudinal direction of axis A. In the illustrated embodimentof FIGS. 1-7, the lower crown part 14 is forged in the longitudinal axisA to form the upper head portion 16, including the inner wall 50, outerskirt wall 53 and the channel 56 therebetween, as well as the floor 66of the inner oil gallery 68 with forging die tools from above. Frombelow, the piston skirt 24 and pin bosses 20 are forged in thelongitudinal direction of the axis A of the piston 10. The skirtportions 26 have an outer working surface 78 which is in line with andformed as an extension of the outer surface of the ring belt 38 when theupper and lower crown parts 12,14 are joined. The outer working surface78 is adapted to confront the walls of a piston cylinder (not shown)during the reciprocating movement of the piston to assist in keeping thepiston aligned properly in the piston bore during operation of theengine (not shown). The outer working surface 78 is preferably parallelto the longitudinal axis A. The skirt portions 26 have a radiallyinwardly facing inner surface 80 which is preferably disposed in anangle relative to the outer working surface 78 such that the skirtportions 26 are relatively thinner near their lower free ends 82 thereofand relatively thicker in the vicinity of the upper skirt wall 53, andpreferably tapers so as to be continually thickening in the direction ofthe lower to upper end of the piston skirt 24. The length of the skirtportions 26 is preferably about the same as and still more preferablyslightly longer than the length of the pin bosses 20, such that thelower end 82 of the skirt portions 26 reside at or slightly below thelower ends 22 of the pin bosses 20.

[0033] Inner surfaces 84 of the pin bosses 20 and the lateral space 86therebetween is likewise formed in the axial forging of the lower crownpart 12 from below along the longitudinal axis A. The inner surfaces 84are preferably disposed at an angle relative to the longitudinal axissuch that the lateral space 86 is wider near the lower ends 22 of thepin bosses 20 and near their upper ends. The inner surfaces 84 arepreferably set at about a 12.5° angle relative of the longitudinal axisso as to further provide the pin bosses 20 with a variable width orthickness as measured along the pin bore axis B in the direction of thelongitudinal axis A, such that the pin bosses 20 are thinner near thelower ends 22 and continually thicken in the direction of the uppercrown part 12 to a point at or above the apex of the pin bores 30. Theinner surface 80 of the skirt portions 26 are preferably set at about a2.5° angle relative to the outer surface 78.

[0034] In the first embodiment, the longitudinal forging of the lowercrown part 14 from below also forms the underside surface 88 of theinner gallery floor 66.

[0035] In addition to forging the lower crown part 14 in the directionof the longitudinal axis A, the lower crown part 12 is further forgedalong an axis transverse to the longitudinal axis A. In the firstembodiment, the lower crown part 12 is forged in the transversedirection of the bore axis B to form transverse forged recesses 90 inthe strut portions 28 of the piston skirt 24. As illustrated in FIGS.1-7, the pin bosses 20 are formed with an outer surface 92 that isspaced radially inwardly of the outer surface of the ring belt 38 andskirt wall 53. The forged recesses 90 extend radially inwardly of theouter surface 92 of the pin bosses 20 and are undercut in thelongitudinal direction such that the recesses 90 could not be formed intheir entirety from forging in the longitudinal direction A of thepiston 10. As shown, the recesses 90 at least partly surround the pinbores 30 and extend laterally inwardly toward the pin bore axis B at alocation above the pin bore axis B and thus are at least partiallyundercut in the longitudinal direction. The upper undercut regions ofthe recesses 90 are indicated at 94 and may extend completely across theface of the pin bosses at a location above the pin bores 30 and belowthe upper skirt wall 53.

[0036] The recesses 90 may also be longitudinally undercut adjacent thelower end of the piston such that a lower wall 94 of the recesses 90 isspaced above a lower end 96 of the struts 28. The recesses 90 mayfurther extend into the skirt portions 26 and may likewise belongitudinally undercut to provide the skirt portions 28 with agenerally I-shaped configuration. The transverse forging operationperformed on the lower crown part 14 reduces the bulk of the material inthe strut portions 28, which is displaced elsewhere to provide materialfor the formation of adjacent features of the lower crown part,including the pin bosses 20 and skirt portions 26. The walls of therecess 90 are preferably spaced from the pin bores 30, leaving athickened hub region 98 surrounding the pin bores 30. The recesses 90extend both below and above an upper apex 100 of the pin bores 30.

[0037] The multi-axial forging of the lower crown part 14, including theformation of the recesses 90, takes place prior to friction welding thelower part 14 to the upper part 12.

[0038] Turning now to FIGS. 8 and 9, an alternative piston constructionis shown generally at 210, wherein the same reference numerals are usedto indicate corresponding features of the piston 10 of the firstembodiment, but are offset by 200. The piston 210 includes upper andlower crown parts 212,214 having mating inner 244,250 and outer 238,253walls which are united across mutual joining surfaces preferably byfriction weld joints 260,262 to define a closed outer oil gallery 264.It will be seen that, unlike the piston 10 of the first embodiment,there is no inner gallery floor 66 or inner oil gallery 68 as in thefirst embodiment. Instead, a space 122 above the inner surfaces 284 ofthe pin bosses 220 extends from the pin bosses 220 upwardly to thecombustion bowl wall 234 in open communication with the lateral space286 between the inner surfaces 284 of the pin bosses 220.

[0039] As shown best in FIG. 9, the space 122 includes an undercut lowerwall surface 124 that extends laterally outwardly from the innersurfaces 284 of the pin bosses 220 in relation to the pin bore axis Band transitions into an upwardly extending side wall 126 of the lowercrown part 214 which is spaced laterally outwardly of the inner surfaces280 of the pin bores 220. The side wall 126 is preferably defined by theradially inner surface of the inner wall 250. The upper crown part 212has an inner side wall 128 that defines an extension of the side wall126 of the lower crown part 212, and is likewise defined by the innersurface of the inner wall 244 of the upper crown part 212. It will beappreciated from FIG. 9 that it would not be possible to forge thepiston 210 as one piece (i.e., with the upper and lower crown parts212,214 formed of one forged piece of the same material without thefriction weld joint, since it would not be possible to form both thenarrowing passage 286 between the pin bores 220 while at the same timeforming the enlarged undercut space 122 above the pin bosses since theundercut regions of the space 122 could not be accessed by forging dietools extended from below through the pin bosses 220.

[0040] By forging the lower crown part 214 separately from the uppercrow part 212 in the general manner illustrated in FIG. 11, it becomespossible to readily form such an undercut space 122 by selection of theappropriate upper die tools in the longitudinal forging direction. Inother words, the upper die tool 110 of FIG. 11 is of a size and shape toform the side walls 126 and lower wall 124 in the lower crown part 214,such that when the upper and lower crown parts 212,214 are frictionwelded together, the enlarged undercut space 122 results, decreasingunneeded mass of material which is displaced elsewhere to form otherstructural parts of the lower crown part 214 to provide for a lighter,more efficient piston 210.

[0041] Like the piston 10 of the first embodiment, the piston 210includes the same unitary piston skirt 224 which may be multi-axiallyforged to include the recesses as previously described. The descriptionand illustration of such features and details are understood and willnot be repeated in regard to the second embodiment 210. Thus, theprinciple difference between the first and second embodiments is thatthe second embodiment lacks the closed central oil gallery and is formedinstead with an oversized, undercut space 122 extending above the pinbosses 220 in open communication with the space 86 between the pinbosses 220.

[0042] Turning now to FIG. 10, a third embodiment of the piston 310 isshown, with the same reference numerals being used to indicate likefeatures in connection with the first embodiment 10, but offset by 300.The piston 310 includes an upper crown part 312 and a lower crown part314 which are friction welded together in the manner previouslydescribed to permanently unite the crown parts into a unitary pistonstructure to define either a single or dual oil gallery structure asdescribed above.

[0043] The lower crown part 314 is formed with pin bosses 330 and apiston skirt 324 which is forged as one piece with the pin bosses 320.Unlike the previous two embodiments, the skirt portions 326 are formedwith at least a pair of opposed upper forged gaps or slots 128 definingupper free edges 130 of the skirt portion 326 which are uncoupled andspaced from the ring belt 338, but nonetheless united by the strutportions 328 to the pin bosses 320 as an integral structure of the lowercrown part 314. The formation of the upper slots 128 takes place in amulti-axial forging operation in forging the lower crown part 314,wherein lateral die tools are brought laterally inwardly either alongthe pin bore axis or perpendicular to the pin bore axis in perpendicularrelation to the longitudinal axis A of the piston 310 during the forgingof the lower crown part 314. The upper slots 128 could be formed aloneor in addition to the multi-axially formed recesses 90 described abovewith respect to the first embodiment 10 of the piston.

[0044] In addition to the upper slots 128, the skirt portions 326 mayfurther be formed with one or more additional slots 132 intermediate theupper and lower ends of the skirt portions 310. One such slot 132 isillustrated in FIG. 9 as being in line with the pin bore axis B. Theslots 128,132 serve to lighten the piston 10 by eliminating material andfurther assist in decreasing oil consumption of an engine by presentingfree edges 130,134 which scrape oil from the walls of a piston cylinderduring operation of the piston. The slots 128,132 further serve to ventthe piston skirt portions 126 and to uncouple them from the head of thepiston in order to isolate the skirt portions 126 from the direct flowof heat from the head and to introduce cooling spaces into the skirtportions 126.

[0045] The disclosed embodiments are representative of presentlypreferred forms of the invention, but are intended to be illustrativerather than definitive thereof. The invention is defined in the claims.

What is claimed is:
 1. A method of forging a piston comprising: dieforging a blank of steel in a first axial direction longitudinally ofthe piston to produce a one piece forged structure including at least aportion of a piston head, a pair of laterally spaced pin bossesextending downwardly from the piston head portion having a common pinbore axis transverse to said first axial direction, and a piston skirthaving a pair of laterally opposed skirt portions spaced from the pinbosses and strut portions extending between and joining said skirtportions to the pin bosses as a one piece structure; and further dieforging the blank in at least a second axial direction transverse to thefirst axial direction to produce a forged recess in each of the strutportions of the piston skirt.
 2. The method of claim 1 wherein the dieforging in the second axial direction is in line with the pin bore axis.3. The method of claim 2 including forming outer surfaces of the pinbosses and forging the strut portions in the second axial direction suchthat the recesses extend laterally inwardly of the outer surfaces of thepin boss portions.
 4. The method of claim 1 wherein the skirt is formedwith a lower edge longitudinally adjacent a lower edge of the pinbosses.
 5. The method of claim 1 including forming pin bores in the pinbosses about the pin bore axis the pin bores having an upper apex. 6.The method of claim 5 wherein the recesses forged in the strut portionsare formed so as to extend below the upper apex of the pin bores.
 7. Themethod of claim 1 wherein the recesses forged in the web portions areformed to extend above and below the pin bore axis.
 8. The method ofclaim 1 wherein the recesses forged in the strut portions extendlaterally inwardly of laterally opposed edges of the pin bores at alocation above the pin bore axis.
 9. The method of claim 8 wherein therecesses are formed to extend laterally across the pin bosses above thepin bores.
 10. The method of claim 1 wherein the strut portions havelower edges and the recesses are each longitudinally undercut such thata lower edge of the recesses are spaced at least in part from the loweredge of the strut portions.
 11. The method of claim 1 wherein a portionof the recesses are formed to extend into the skirt portions.
 12. Themethod of claim 11 wherein the skirt portions are formed with a loweredge and the extend recess portions are formed so as to be spaced fromthe lower edge of the skirt portions.
 13. The method of claim 1 whereinthe skirt is joined directly to the piston head as a one piecestructure.
 14. The method of claim 1 wherein the head is formed with atleast one closed oil gallery.
 15. The method of claim 14 includingforming the piston head in at least two parts and joining them togetherto define a closed oil gallery within the head.
 16. The method of claim15 including forging at least a portion of the gallery in the firstaxial forging operation.
 17. The method of claim 15 including joiningthe piston head parts by friction welding.
 18. The method of claim 15including forging at least one oil drain access opening.
 19. The methodof claim 18 wherein the access opening is formed to open to an oil ringgroove.
 20. The method of claim 1 including forging the skirt portionswith an inner wall that is tapered in the longitudinal direction suchthat the skirt portions are thicker at an upper region thereof adjacentthe head and are narrow adjacent a lower end of the skirt portions. 21.The method of claim 1 including forming the piston to include an uppercrown part and a lower crown part, each part having a circumferentiallyextending inner wall and a circumferentially extending outer wall spacedradially outwardly from said inner wall thereof, the outer wall of theupper part being defined by a ring belt and the outer wall of the lowerpart being defined by the piston skirt.
 22. The method of claim 21including forming the inner walls with matable joining surfaces andfriction welding the inner walls together across the joining surfaces tounite the upper and lower crown parts.
 23. The method of claim 22including forming the outer walls with matable joining surfaces andfriction welding the outer walls together across the joining surfacesthereof simultaneously with the friction welding of the inner walls tounite the piston skirt to the ring belt.
 24. The method of claim 23including forging a circumferential recess between the inner and outerwalls of the upper and lower crown parts so as to form a closed galleryupon friction welding the crown parts together.
 25. The method of claim23 including forming at least one ring groove in the outer wall of thelower crown part below the friction weld joint of the outer walls. 26.The method of claim 23 including forming the joining faces of the innerand outer walls such that the friction weld joints of the inner andouter walls lie in different planes.
 27. The method of claim 23including forming the lower crown part with an inner gallery floorspanning said inner wall thereof and an outer circumferentiallyextending gallery floor between the inner and outer walls so as to forminner and outer closed oil galleries between said crown parts uponfriction welding the crown parts together.
 28. The method of claim 24including forging at least one oil drain access hole in the lower crownpart.
 29. The method of claim 1 including forming in the forging processat least a lower crown part of the piston having the piston skirt andpin bosses formed as one piece, and including forming the pin bosseswith inner faces separated by a lateral space during the first axialforging step and forging in the same step a cavity located above the pinbore axis in open communication with the lateral space which is undercutin relation to the inner faces of the pin bores in the longitudinaldirection away from the bottom of the pin bosses such that the cavityextends laterally outwardly of the inner pin boss faces in the directionof the pin bore axis.
 30. The method of claim 29 including forming anassociated upper crown part and friction welding the upper and lowercrown parts together across a friction weld joint.
 31. The method ofclaim 30 wherein the upper and lower crown parts are formed such thatthe friction weld joint passes through the cavity.
 32. The method ofclaim 31 wherein the upper and lower crown parts are formed such thatthe cavity extends above and below the friction weld joint.
 33. Themethod of claim 30 including forming a closed oil gallery between thecrown parts upon joining the crown parts together while surrounds thecavity and is separated therefrom by adjoined inner walls of the crownparts.
 34. The method of claim 29 including forging the inner faces ofthe pin bosses at an angle to provide a variable width of the pin bossesincreasing in the longitudinal direction away from the lower end of thepin bosses.
 35. A forged piston comprising: a piston head having acombustion bowl and a ring belt with a plurality of ring grooves formedtherein; a pair of pin bosses extending downwardly from said piston headhaving pin bores formed therein aligned along a common pin bore axistransverse to a longitudinal axis of the piston head; a piston skirtforged as one piece with said pin bosses including a pair of opposedskirt portions spaced from said pin bosses and intervening strutportions extending between and uniting said skirt portions to said pinbosses, said strut portions presenting outer surfaces facing in oppositedirections along said pin bore axis and having lower edges; and forgedrecesses formed in said outer surfaces of said strut portions.
 36. Thepiston of claim 35 wherein said pin bores have laterally opposed edgesand said recesses extend laterally inwardly of said pin bore edges at alocation above said pin bore axis.
 37. The piston of claim 36 whereinsaid recesses extend laterally across said pin bosses above said pinbores.
 38. The piston of claim 35 wherein said pin bosses have outerfaces and said recesses extend inwardly of said pin bore faces.
 39. Thepiston of claim 35 wherein strut portions have lower edges and saidrecesses each have a longitudinally undercut lower edge that is spacedat least in part from said associated lower edge of said strut portion.40. The piston of claim 35 wherein a portion of said recess extends intosaid skirt portions of said skirt.
 41. The piston of claim 40 whereinsaid skirt portions have a lower edge and said extended portion of saidrecess is spaced from said lower edged of said skirt portions.
 42. Thepiston of claim 35 wherein said recesses are symmetrical across alongitudinal plane containing both said longitudinal axis of said pistonand said pin bore axis.
 43. The piston of claim 42 wherein each of saidrecesses extend across said longitudinal plane above said pin bores. 44.The piston of claim 35 wherein said piston head includes at least oneoil cooling gallery.
 45. The piston of claim 44 wherein said piston headincludes at least one friction welded joint.
 46. The piston of claim 45wherein said gallery has a closed bottom end.
 47. The piston of claim 35wherein said piston skirt has an upper end and said ring belt has alower end.
 48. The piston of claim 47 wherein said upper end of saidpiston skirt is coupled to said lower end of said ring belt to define atleast one closed oil cooling gallery within said piston head.
 49. Thepiston of claim 47 wherein said piston skirt is friction welded to saidring belt.
 50. The piston of claim 35 wherein said piston is fabricatedof steel.
 51. The piston of claim 35 wherein said piston head includesan upper crown having said ring belt defining an outer circumferentiallycontinuous wall having a lower joining surface and a circumferentiallycontinuous inner wall spaced radially inwardly of said ring belt andhaving a lower joining surface, and a lower crown having said pistonskirt defining an outer circumferentially continuous wall having anupper joining surface and a circumferentially continuous inner wallspaced radially inwardly of said piston skirt and having an upperjoining surface, and whereby said upper and lower joining surfaces ofsaid inner and outer walls are joined by friction welding to define atleast one closed oil cooling gallery.
 52. The piston of claim 51 whereinsaid piston head includes a radially inner and radially outer coolinggallery separated by said inner wall.
 53. The piston of claim 51 whereinsaid piston head includes at least one oil drain opening.
 54. The pistonof claim 51 wherein said lower crown includes a circumferential recessbetween said inner and outer wall extending below said upper joiningsurfaces of said inner and outer walls to define a lower portion of saidcooling gallery residing below said friction weld joint and extendinginto said lower crown.
 55. The piston of claim 54 wherein said outerwall of said lower crown is formed with at least one of said ringgrooves residing below said friction weld joint of said outer walls ofsaid upper and lower crowns.
 56. The piston of claim 54 wherein saidcircumferential recess extends into said skirt portions of said lowercrown.
 57. The piston of claim 51 wherein said joining surfaces of saidouter walls are spaced above said joining surface of said inner walls ina different plane.
 58. The piston of claim 35 wherein said pin bossesinclude inner faces separated by a lateral space and a forged cavitylocated above said pin bores in open communication with said space andbeing undercut in said pin bosses so as to extend laterally outwardly ofsaid inner faces of said pin bosses in the direction of said pin bossaxis.
 59. The piston of claim 58 wherein said piston head includes anupper crown and a lower crown part having adjoined inner walls thereofwith an inner surface spaced laterally outwardly of said inner faces ofsaid pin bosses and defining a longitudinally extending inner surface ofsaid cavity, said inner walls having a joint passing through saidcavity.
 60. The piston of claim 58 wherein said joint comprises afriction weld joint.
 61. The piston of claim 58 wherein said innersurface of said inner walls extend vertically upwardly or at a positiveoutward draft angle from said inner faces to said joint.
 62. The pistonof claim 58 wherein said cavity extends into said upper crown above saidjoint in said undercut relation to said inner faces of said pin bosses.63. The piston of claim 58 wherein head includes a closed oil gallerysurrounding said cavity and separated therefrom by said inner walls. 64.The piston of claim 58 wherein said inner faces are set at an angle suchthat said pin bosses have a varying width longitudinally of said pistonbetween a lower end of said pin bosses and an upper region above saidpin bosses adjacent said cavity.
 65. The piston of claim 64 wherein saidwidth of said pin bosses increases continuously across said pin bores.66. The piston of claim 64 wherein said space between said inner facesdecreases in the longitudinal direction from said lower end of said pinbosses toward said cavity.
 67. The piston of claim 35 wherein saidpiston skirt portions are tapered so as to increase in thickness in thelongitudinal direction of said piston from a lower end of said skirtportions toward said piston head as measured in a longitudinal planeperpendicular to said pin bore axis.
 68. The piston of claim 67 whereinsaid piston skirt portion includes an outer wall and an inner wall, saidinner wall being set at an angle with respect to said outer wall toprovide a continuously increasing said thickness in the longitudinaldirection away from said lower end of said skirt portions.
 69. Thepiston of claim 68 wherein said angle is set at about 2° or greater. 70.The piston of claim 68 wherein said angle is set at less than 3°.
 71. Amono steel piston comprising: an upper crown portion fabricated of steelhaving a head portion formed with at least one circumferentiallyextending mating surfaces; and a lower crown portion forged from asingle piece of steel including an upper head portion having at leastone circumferentially extending mating surface, a pair of pin bossportions extending downwardly from said upper head portion in laterallyspaced relation to one another; and an integral skirt formed as a singleforged piece with said pin boss portions including a pair of opposedskirt portions spaced from said pin boss portions and strut portionsextending between and intermediate said skirt portions and said pin bossportions; said mating surface of said lower crown portion being frictionwelded to said mating surface of said upper crown portion.
 72. Thepiston of claim 71 wherein said piston skirt portions include an upperfree edge spaced and discoupled from said upper crown portion.
 73. Thepiston of claim 71 wherein said upper crown portion includes a pair ofsaid circumferentially continuous joining surfaces spaced radially fromone another, and said lower crown portion includes a corresponding pairof said circumferentially continuous joining surfaces friction welded tosaid joining surface of said upper crown portion to define at least oneclosed oil gallery therebetween.
 74. The piston of claim 73 wherein saidfriction welded crown portions include at least two closed oilgalleries.
 75. The piston of claim 73 wherein said piston skirt portionsinclude an upper free edge spaced and discoupled from said upper crownportion.
 76. The piston of claim 71 wherein said lower crown portion ismulti-axially forged to include forged recesses in the piston skirt inone or more areas inaccessible by forging in the longitudinal directionof the piston.
 77. A method of making a piston comprising: fabricatingan upper crown portion of steel having a head portion with at least onecircumferentially continuous joining surface; and forging a lower crownportion from a single piece of steel including an upper head portionhaving at least one corresponding circumferentially continuous joiningsurface, a pair of pin boss portions extending downwardly from the upperhead portion, and a piston skirt forged as one piece with the pin bossportions, and friction welding the joining surfaces of the upper andlower crown portions to unite the upper and lower crown portions.